In a WDM transmission system, two or more optical data carrying channels, each defined by a different carrier wavelength, are combined onto a common path for transmission to a remote receiver. The carrier wavelengths are sufficiently separated so that they do not overlap in the frequency domain. The multiplexed channels are demultiplexed at the receiver in the optical and possibly also in the electrical domain. Demultiplexing in the optical domain requires using frequency-selective components such as optical gratings or bandpass filters. Typically, in a long-haul optical fiber system, an optical amplifier would amplify the set of wavelength channels simultaneously, usually after traversing distances less than about 120 km.
One class of optical amplifiers is rare-earth doped optical amplifiers, which use rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica fiber core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimal. Also, the erbium-doped fiber amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression. EDFAs are also attractive because they are fiber devices and thus can be easily connected to telecommunications fiber with low loss.
FIG. 1 shows an energy level diagram for the Er+3 system. As shown, light of wavelength 980 nm is absorbed by the erbium ions, exciting the ions to the higher energy state 4I11/2. This excited state rapidly decays (with a time constant τ32 of about 10 microseconds) to the metastable state 4I13/2 without radiative emission. The metastable state alternatively may be reached by the absorption of light at 1480 nm, which corresponds to the upper edge of the band defining the metastable state. The metastable state deexcites by emitting photons at different wavelengths, with peak photon emission occurring at about 1530 nm. This deexcitation may occur spontaneously or by stimulated emission with an optical signal having a wavelength around 1530 nm. Since the metastable state is relatively long-lived (with a time constant τ21 of about 10 milliseconds), stimulated emission is much more likely to occur than spontaneous emission under typical operating conditions. Stimulated emission causes amplification of the optical signal, which induced the stimulated emission.
The signal power directed to the input of an optical amplifier employed in an optical communication system can vary for a large number of reasons. For example, power variations can be caused by an intentional increase or decrease in the number of channels for the purpose of routing traffic, by the unintentional loss of channels due to a fiber cut or human error, changes in span losses, and component loss changes due to aging or temperature fluctuations. FIG. 2 shows how the amplifier gain typically varies with the total input signal power. The gain at small input signal power levels is known as the small-signal gain. As the input power is increased, the gain of the amplifier begins to decrease. When the amplifier is generating less than its small-signal gain, it is said to be operating in saturation. Saturation of the amplifier typically occurs when the signal power within the amplifier becomes a significant fraction of the pump power, leading to pump depletion and a reduction in the amplifier gain. In a heavily saturated amplifier, the total output signal power will remain roughly constant even while the total input signal power changes. For instance, if N channels located at different wavelengths each having an equal power are initially injected into a heavily saturated, gain-flattened amplifier, and then the number of channels is suddenly reduced to one, the total output power from the amplifier will remain constant. Therefore, the output power of the remaining channel will have increased from its initial value by approximately by a factor of N.
Since amplifier gain is designed to offset loss in a transmission system, it is important to maintain a constant amplifier gain as the input power changes. This type of control is commonly referred to as automatic gain control (AGC) or transient control. It is well known that AGC can be achieved by adjusting the pump power supplied to the amplifier. In general, the required change in pump power depends not only on the input signal power level but also on the spectral content of the input signal.
Well known techniques for implementing AGC by controlling pump power include feed-forward and feedback arrangements. In a feed-forward arrangement the pump power is adjusted based solely upon changes to the input signal. Typically, the feed-forward pump power adjustment is made based on a change in the aggregate input-signal power. While feed-forward arrangements offer the advantage of a fast response time, they can also be inaccurate, since the appropriate pump power is a function of not only the input power but also the spectral content of the input signal. Moreover, the accuracy of a feed-forward arrangement generally diminishes over time because as the various components of the amplifier age their characteristics often change.
In a feedback arrangement, the parameters used to determine the appropriate pump power include at least one output parameter, and no changes are made to the pump power based solely upon changes to the power of the input signal. For instance, the input and output optical signals may be detected and used to determine the actual gain of the amplifier. This measured gain may then be used to adjust the pump power until the desired gain is achieved. For example, FIG. 3 shows an optical amplifier with such a feedback control. The arrangement of FIG. 3 comprises an erbium doped fiber 1, a pump laser 2, a wavelength multiplexer 3 which multiplexes the pump laser output and an input optical signal which is to be amplified and is input at port 4, an input signal tap 12, which serves to split off a small portion of the input signal to doped fiber 1, an output signal tap 5, which serves to split off a small portion of the output signal, an output port 6 for receiving the amplified optical signal, detectors 8 and 14, electronic amplifiers 9 and 16 and a feedback circuit 10.
In operation, the optical signal to be amplified is input via port 4 of multiplexer 3, multiplexed with the optical pump signal output from laser 2 and amplified in the erbium doped fiber 1. Tap 12, which may be a fused fiber coupler, for example, splits off a small proportion of the signal input to the fiber 1. This small part of the amplified signal, which is employed as a control signal, is detected by detector 14, amplified by electronic amplifier 16 and applied to the feedback circuit 10. Likewise, tap 5, which may also be a fused fiber coupler, for example, splits off a small proportion of the amplified signal output from fiber 1. This small part of the amplified signal, which also serves as a control signal, is detected by detector 8, amplified by amplifier 9 and applied to the feedback circuit 10. Feedback circuit 10 determines the amplifier gain based on the two control signals it receives. The output from the feedback circuit 10 is applied to the pump laser 2 and serves to vary the pump laser 2 output power to maintain constant gain.
One variant of the feedback arrangement shown in FIG. 3 employs the amplified spontaneous emission (ASE) rather than the signal itself as the control signal. As is well known, all optical amplifiers generate ASE. As shown in FIG. 4, conventional “C-band” erbium amplifiers provide substantial gain in the range of 1529-1564 nm. Likewise, the ASE is strongest over this same wavelength range because the ASE power is directly proportional to amplifier gain. That is, the intensity of the amplified spontaneous emission from the amplifier is dependent on amplifier gain, and thus a measure of ASE provides an indirect measure of the amplifier gain. Accordingly, one or more wavelengths within the 1529-1564 range may be reserved for measuring ASE at that wavelength. For example, in FIG. 4, ASE is measured at a wavelength of 1.551 microns, which can therefore be used to form the basis of a gain control loop of the form illustrated in FIG. 5.
In FIGS. 3 and 5, like elements are denoted by like reference numerals. In FIG. 5, however, coupler 5 is now a wavelength selective coupler that splits off a small portion of the ASE. Thus in this arrangement amplifier gain is monitored by monitoring the ASE over the wavelengths demultiplexed by coupler 5, which is used by the feedback loop to keep the amplifier gain constant by varying the pump power accordingly.
The previously discussed feedback arrangements for providing an optical amplifier with AGC have a number of advantages and disadvantages. For instance, one advantage of the first approach in which the signal itself is used as the control signal is that it can use broadband input and output couplers, which are simple and inexpensive components. However, a feedback system that uses a portion of the amplified signal as the control signal is only accurate if the gain of the optical amplifier is wavelength-independent. On the other hand, while a feedback approach that uses the ASE as the control signal can be used with an optical amplifier having a wavelength-dependent gain, it requires a relatively expensive filter to de-multiplex the ASE from the output signals and the ASE level may not be directly proportional to gain in some amplifier designs.
Moreover, all feedback approaches have one disadvantage in common: they cannot respond to a transient change in the input power until a disruption in the performance of the amplifier is measured. In other words, the response time of the AGC is limited by the latency of the EDFA itself. This disadvantage is particularly troublesome when a very large change in the input power occurs on a timescale that is much faster than the response time of the amplifier, i.e. ˜1 μs for a typical saturated telecommunications amplifier
Accordingly, there is a need for an optical amplifier having an improved automatic gain control arrangement that is both accurate and fast to respond.